Internet routers in communication systems receive internet protocol (IP) packets at interfaces on line cards. A router contains numerous line cards, each of which can have a number of interfaces. Typically, each interface has an associated access control list (ACL) stored on the line card within a Ternary Content Addressable Memory (TCAM). An ACL is a set of rules to be applied to IP packets in order to filter unwanted packets, or perform other actions on packets such as counting or copying. Each rule is composed of a key and an action. When an IP packet arrives through an interface, the router extracts specific fields from the packet to form a key and searches the ACL associated with the interface for a rule having a matching key. If a rule is found, the filter applies the action associated with the rule to the packet.
Interfaces may have identical sets of rules. Some routers take advantage of this to simplify configuration and troubleshooting, and to improve robustness. Sets of rule are copied between ACLs. However, once loaded into the TCAM, each rule of each ACL of each interface is stored and accessed separately. There is therefore no saving of TCAM storage space. Other routers go further and share a single ACL between two or more interfaces. While this somewhat reduces the amount of TCAM storage space needed, this is only possible if the interfaces have all their rules in common.
Improvements in the storage capacity of TCAMs has not progressed as quickly as improvements in access rate. Due to space and power constraints associated with TCAMs and to the cost of TCAMs, it is preferable to keep the number and size of TCAMs on each line card to a minimum. This may be difficult in some situations, as routers often support hundreds of interfaces and hundreds of ACLs, requiring large TCAM storage space. Line interface cards which minimized the total number of rules being stored would allow fewer or smaller TCAMs to be used, thereby saving space and power. Alternatively, such cards could support more rules for the same space and power usage.